This invention relates to the blocking of semi-finished lens blanks, and, more particularly, to the precision attachment of blocks to the surfaced sides of semi-finished lens blanks with multi-focal segments to permit prescribed surfacing of the opposite sides of the blanks.
Semi-finished lens blanks require further processing before they can be used. Such blanks often incorporate one or more special elements, such as bifocal segments. One side of the lens containing, for example, a bifocal segment, is ground i.e., surfaced, in a prescribed fashion. The opposite side is in a roughened form just as it is received from the mold.
Before a semi-finished lens blank with a bifocal segment can be used, the rough side must be surfaced in accordance with a prescription provided by an oculist or optometrist. This requires the placement of a temporary hub or block on the finished side of the blank so that it can be held by an appropriate tool for finishing the other side, as well as the edge of the blank.
In the usual procedure the desired optical parameters are manually located and marked on the blank. A representative device for marking a lens blank is described in U.S. Pat. No. 2,917,971 which issued to L. W. Goddu, et al. on Dec. 22, 1959. Once the lens blank has been appropriately marked, it is then transferred to a special instrument or "blocker" where the hub or block is temporarily affixed. This procedure is cumbersom and inaccurate. It requries operators with considerable skill, yet it typically produces tolerances which do not satisfy recognized standards. The consequence, for example, of improper location of the optical center of a multifocal lens can be eye strain, headaches and other visual difficulties.
Accordingly, it is an object of the invention to increase the precision with which lens blanks can be processed, particularly lens blanks with special elements. A related object is to obtain increased precision in the processing of multifocal lens blanks. Another related object is to achieve the precise blocking of lens blanks, particularly bifocal and multifocal lens blanks.
Another object is to reduce the number of rejects encountered in the processing and blocking of lenses, especially multifocal lenses. A companion object is to raise the level of accuracy of acceptable lens blanks.
A further object is to achieve the precision processing of lens blanks with only semi-skilled operators. A related object is to facilitate the finishing of semi-finished lens blanks, particularly those containing multifocal segments.